1. Field of the Invention
This invention relates in general to electric submersible pumps (ESPs) and, in particular, to a high efficiency impeller for use in an ESP.
2. Brief Description of Related Art
Electric submersible pump (ESP) assemblies are disposed within wellbores and operate immersed in wellbore fluids. ESP assemblies generally include a pump portion and a motor portion. Generally, the motor portion is downhole from the pump portion, and a rotatable shaft connects the motor and the pump. The rotatable shaft is usually one or more shafts operationally coupled together. The motor rotates the shaft that, in turn, rotates components within the pump to lift fluid through a production tubing string to the surface. ESP assemblies may also include one or more seal sections coupled to the shaft between the motor and pump. In some embodiments, the seal section connects the motor shaft to the pump intake shaft. Some ESP assemblies include one or more gas separators. The gas separators couple to the shaft at the pump intake and separate gas from the wellbore fluid prior to the entry of the fluid into the pump.
The pump portion includes a stack of impellers and diffusers. The impellers and diffusers are alternatingly positioned in the stack so that fluid leaving an impeller will flow into an adjacent diffuser and so on. Generally, the diffusers direct fluid from a radially outward location of the pump back toward the shaft, while the impellers accelerate fluid from an area proximate to the shaft to the radially outward location of the pump. Each impeller and diffuser may be referred to as a pump stage.
The shaft couples to the impeller to rotate the impeller within the non-rotating diffuser. In this manner, the stage may lift the fluid. The impeller includes vanes circumferentially spaced around the impeller. The vanes may be straight or curved. The vanes will define passages through which fluid may move within the impeller. The vanes may push fluid from the radially inward fluid inlet to the radially outward location, pressurizing the fluid. Maximum pump efficiency generally occurs at a particular flow rate or along a range of flow rates, where the range is typically significantly less than the operating range of flow rates. Pumps are usually designed to operate at or close to a maximum efficiency. However, fluid flow rates through a pump may change, such as due to depletion of fluids in a reservoir, so that over time a pump may not be operating at its maximum efficiency. A key factor in pump efficiency is the prevention of fluid boundary separation from the impeller vane. Fluid boundary separation may occur as the speed of the impeller rotation increases. When the fluid boundary separates from the surface of the impeller vane, turbulent flow is introduced, increasing drag and thus, decreasing the acceleration imparted to the fluid from the impeller vane. This decreases pump efficiency and leads to an increase in pump energy requirements. Therefore, an impeller vane that could decrease the instances of fluid boundary separation from the impeller vane and consequently increase efficiency would be desired.